The present invention relates to an objective lens for an optical pick-up that is capable of reading/writing information data from/onto a plurality of kinds of optical discs whose working wavelengths are different from each other. Particularly the present invention relates to the objective lens that includes a refractive lens and a diffractive lens structure formed on a surface of the refractive lens.
There are several types of optical discs whose working wavelengths are different from each other. For instance, the working wavelength for a CD (compact disc) or a CD-R (CD recordable) is about 780 nm, while that for a DVD (digital versatile disc) is about 650 nm. Further, a new standard called an HD-DVD whose recording density is higher than that of a DVD is proposed. The working wavelength for an HD-DVD is about 400 nm because of its high recording density.
At any wavelengths chromatic aberration should be reduced as small as possible to resist a variation of an emission wavelength of a light source. Since the dispersion of lens material becomes larger in a short-wavelength region about 400 nm in particular, correction of chromatic aberration is an essential condition in this region.
There are two approaches to correct chromatic aberration. One approach is to employ a combination of a plurality of lenses having different dispersions. The other approach is to form a diffractive lens structure on a surface of a refractive lens. The later approach is preferable for an objective lens of an optical pick-up from the viewpoints of cost, weight and size.
However, when the objective lens is used at two or three working wavelengths including that for an HD-DVD, a range of the working wavelength increases remarkably, which increases loss of light quantity. Namely, since diffraction efficiency of a predetermined diffraction order varies in response to changes in wavelength, it is impossible to keep the high diffraction efficiency for all wavelengths among the wide range of the working wavelength.
Further, dispersion (dn/dxcex) of refractive material at a wavelength of about 400 nm is several times larger than that at a wavelength of about 650 nm that is the working wavelength of a DVD. That is, the chromatic aberration of the refractive lens in the short-wavelength region is larger than that in the middle- or long-wavelength region. On the other hand, the power of the diffractive lens structure is proportional to a wavelength. That is, the correction effect in chromatic aberration of the same diffractive lens structure becomes larger as the wavelength increases. As a result, the objective lens that is corrected in chromatic aberration at a wavelength of about 400 nm cannot be used as an objective lens to write information data onto a DVD because of its over-corrected chromatic aberration.
It is therefore an object of the present invention to provide an objective lens for an optical pick-up, which is able to correct chromatic aberration with keeping high diffraction efficiency even when the range of the working wavelength is relatively wide.
For the above object, according to the present invention, there is provided an improved objective lens for an optical pick-up that converges light beams of at least two different wavelengths onto recording layers of optical discs of different standards, respectively, which includes a refractive lens having a positive power and a diffractive lens structure that is designed such that the diffraction order where the diffraction efficiency is maximized at the shorter wavelength is different from the diffraction order where the diffraction efficiency is maximized at the longer wavelength. The diffractive lens structure has a plurality of concentric ring areas with minute steps at the boundaries therebetween and is formed on at least one surface of the refractive lens.
With this construction, chromatic aberration can be corrected at both the shorter and longer wavelengths, and the diffraction efficiency can be kept high, which reduces loss of light quantity even when the range of the working wavelength is relatively large.
The diffraction order where the diffraction efficiency is maximized at the shorter wavelength is preferably higher than the diffraction order where the diffraction efficiency is maximized at the longer wavelength. The effect for correcting chromatic aberration increases as the diffraction order becomes higher. For instance, the correcting effect of the n-th order diffracted beam is n-times that of the first order diffracted beam.
On the other hand, the correction effect increases as the wavelength becomes longer as described above. Therefore, when the diffraction order of the light beam at the shorter wavelength is higher than that at the longer wavelength, the effects for correcting the chromatic aberration at the shorter wavelength and the longer wavelength can be balanced. For instance, the diffractive lens structure maximizes the diffraction efficiency of the third order diffracted light at the shorter wavelength and maximizes the diffraction efficiency of the second order diffracted light at the longer wavelength.
The shorter wavelength may be about 400 nm that is the working wavelength of an HD-DVD and the longer wavelength may be about 650 nm that is the working wavelength of a DVD. The longer wavelengths may further include about 780 nm that is the working wavelength of a CD or a CD-R.
According to another definition of the present invention, there is provided an improved objective lens for an optical pick-up that converges light beams of at least two different wavelengths onto recording layers of optical discs of at least two different recording densities, respectively, which includes a refractive lens having a positive power and a diffractive lens structure that is formed on at least one surface of the refractive lens to correct chromatic aberration of the refractive lens. The diffractive lens structure is designed such that the diffraction order where the diffraction efficiency is maximized at the working wavelength of the optical disc having high recording density is different from the diffraction order where the diffraction efficiency is maximized at the working wavelength of the optical disc having low recording density.
In general, the working wavelength of the optical disc having high recording density is lower than the working wavelength of the optical disc having low recording density. Therefore, the diffraction order where the diffraction efficiency is maximized at the working wavelength of the optical disc having high recording density is higher than the diffraction order where the diffraction efficiency is maximized at the working wavelength of the optical disc having low recording density.